Some geotechnical properties of offshore sediments

Some geotechnical properties of offshore sediments

406 SUMMARY REVIEWS strong tidal currents. Current eddies associated with Carnsore Point may allow fine sand to accumulate in its lee ; in addition,...

239KB Sizes 10 Downloads 42 Views

406

SUMMARY REVIEWS

strong tidal currents. Current eddies associated with Carnsore Point may allow fine sand to accumulate in its lee ; in addition, the numerous longitudinal sand banks on this platform may form their own localised sand transport systems that might entrap finer sediment closer to shore. The gravelly sea-floor is host to a variety of carbonate-producing macro and micro fauna, although molluscs appear to be dominant (Dobson , Evans & James , 1971). The percentage of carbonate in both the

sand and gravel fractions is relatively high, with over 40% common in the Channel. The sand sheets of Cardigan Bay appear to have a relative dearth of epifauna (Haynes, 1964), and here carbonate sand content is < 10 % . The muddy sediment depressions have a higher carbonate sand content (20-30 % ), principally because they act as sinks for Foraminifera tests and shell fragments (Atkinson, 1971). This summ ary is published with the permission of the Director, British Geological Survey (NERC).

References ATKINSON , K. 1971. The relat ionship of Recent foramin ifera to the sedimentary facies in the turbulent zone, Cardigan Bay. J. nat. Hist., 5, 385-439. BRITISH GEOLOGICAL SURVEY. (In press) . Cardigan Bay . Sheet 52°N-06°W. I ;250,000 Series . Sea Bed Sediments. DOBSON , M. R ., W. E. EVANS & K. H. JAMES . 1971. The sedimenl on the floor of the southern Irish Sea. Mar. Geol., 11,27-69. FOLK, R. L. 1954. The distinction between grain size and mineral composition in sedimentary-rock nomenclature . J. Geol., 6Z, 344-59.

GARRARD , R . A. 1977. The sediments of the South Irish Sea and Nymphe Bank area of the Celtic Sea. In (Kidson, C. & M. J . Tooley; eds.), The Quaternary history of the Irish Sea, SeeI House Press, Liverpool, 69-92. HA YNES, J. R. 1964. Live and dead foraminifera between the Sarns, Cardigan Bay. Nature, 204, 774. HOWARTH, M. J. (In press). In (British Geological Survey) , Cardigan Bay . Sheet 52°N-06 °W. I :250,000 Series. Sea Bed Sediments. MOORE, J. R. 1968. Recent sedimentation in northern Cardigan Bay, Wales. Bull . Brit. Mus. nat. Hist. (Mineral), Z, 21-131.

Some geotechnical properties of offshore sediments S. E. Shackley

Department of Oceanography , University College of Swansea, Singleton Park, Swansea SA2 BPP Recent work (Collins , Pattiaratchi, Banner & Ferentinos, 1980; Shackley, 1982; Shackley & Collins, 1984) has shown that the mobile, surficial soft sediments in the high energy environment of Swansea Bay are transported under the effects of tidally- and storm wave-generated currents. The sands and muds form a layer 1 m thick, on average, over relict F1andrian deposits. Their distribution is highly variable temporally and spatially, sand inputs being associated mainly with storm-generated wave action whilst mud inputs occur mainly by dredged spoil disposal. This variability has meant that in the past research into the processes in operation (sediment dynamics, benthic infaunal community distributions) in the area has been time consuming and labour intensive. A more rapid, remote, i.e. in situ. method to determine sediment type and bottom dynamics together with selected geotechnical properties (Atterberg limits) of the sediment would be of great value. A new instrument designed for the simple and rapid in situ determination of sediment type and bottom dynamics , the Borg Instruments Sediment Cone Penetrometer (Hakanson & Jansson , 1983), is now available. This instrument has been tested in Swansea

Bay and a preliminary attempt has been made to extend the capability of the instrument to include an assessment of the geotechnical properties of the sediment. The Borg Sediment Penetrometer has three cones, LI> the widest, Lz, and, L 3 , the most pointed , which are released at the sediment surface. The depth of penetration of the cones, following a 'settling period' of 5 seconds, is measured in centimetres. Using the calibration table provided (Table 1), the penetration depth of the cones can be converted directly into eight different sediment types, from very hard, rock bottoms to very fine sediments, and into three bottom dynamic types : viz . (i) erosion-areas where particles finer than medium silt (0.006 mm) are not deposited; (ii) transportation-areas with discontinuous deposition of fines; and (iii) accumulation-areas where fine materials are constantly being deposited . Typical values for the water content of the surficial layer (0-1 ern) are given as is a note to the effect that hard sediment type often gives irregular cone penetration depths, whilst fine sediments always give regular, i.e., in sequence Lt-L] , penetration depths. Tests in the study area (Table 2) indicate that the

407

SL:MMA R Y R E VI EWS

TAB LE 1. Calibration table for sediment penetrom eter, Borg System Bottom dynamic type

Sedimen t type Rock, boulder Pebbles , mussels G ravel, sand Sand Sandy silt Hard clay Fine sediments (silty) Very fine sedi ments (mud dy)

Water conte nt ( %)

Erosion Erosion Erosion Erosion Transportation Transpo rtation

0 < 10 10-20 20-50 50-80 50-60

Accumu lation Accumulation

Cone penetrat ion dept h (em)" L2 L1 L, 5

Notes

0- 1 < 1.5 <3 <2

0 2.5 0.5-2.5 1-4 3-10 3-5

2 0 1-5 1.5-7 5-15 5-10

Irregular Irregular Regular Regular Regular Regular

75-85

1.5-4

6- 15

8-20

Regular

85-99

>2

>15

> 20

Regular

I

" Recommend ed that three deter minations are made and the mean values used to classify the sediments

TABL E 2. Sediment Penetrometer and day Grab Survey, Swansea Bay

Statio n

Decca position

Cone penetra tion (em) Ll L} i,

Day grab field observatio n

Penetrometer calibration (from Table I)

I

(Mumbles)

0 46.6 B62.5

0.9

2.9

4.0[6]

2 (Swansea Docks approach jetty)

0 44.5 B68.5

0.3

57.1

65.6[9]

3 (Port Ta lbot Tidal Harbour)

040.0 B74.0

0.4

5.6

6.7[41

4 (Central Swansea Bay)

D42.4

B63.4

0.6

12.4

17.3[6]

Sand , coal, pebbles . shell debris .

Regular Gravel/sand ; sand Erosio n

Recently deposited, loosely consolidated mud.

Regular Very fine sediment s (muddy) Accumulation

Sandy mud

Regular Sandy silt Transportation

Layered: sand on mud on sand

Regular Either : Sandy silt Or:

5 (Sout hwest Swansea Bay)

0 43.0 B54.0

0.6

5.9

7.0[4]

Sand , shell debris Hydroid s.

Transport ation Fine sediments (silty) Accumulation

Regular Sandy silt/Hard clay Transportation

[ ] = Number of penetrometer determinations to obtai n mean values for cone penetrations used to classify sediments

!; 00

TABLE 3. Sediment penetrometer and day grab , Swansea Bay, induding Atterburg limits

Station

Decca position

Cone penetration (em) L3 L1 Lz

1 Port Talbot Tidal Harbour

0 40.0 B73.0

2.1

4.9

8.1[5]

2 Central Swansea Bay

D41.0 B69.0

2.0

6.9

17.6[5]

3 Southwest Swansea Bay

041.0

B65.0

1.5

11.2

11.4(5)

Day grab field observation

Penetrometer calibration (from Table 1)

M

Atterburg limits LL PL PI

LI

Liquid mud over black mud

Regular Sandy silt Transport ation

88

89

39

50

0.98

Sandy mud

Regular Either: Fine sediments (silty) Accumulation Or : Sandy silt T ransportation Regular Fine sediments (silty) Accumulation.

60

59

25

34

1.0

Sand on mud

[ ] = Number of penetrometer determ inations to obt ain mean values for cone penetrations used to classify sediment Atterburg Limits: Metho ds BS1377: 1975. M = moisture content: Test l (A ); Sta ndard method (oven drying). LL = liquid limit: Test 2(C) ; On e point/Casagrande apparatus. PL = plastic limit: Test 3; 3 mm diameter thr ead . PI = plasticity index. LI = liquidity index.

til

c

:::

:::

> -<

lC lC

35

35

19

16

1.1

m

<

m ~

til

409

SUMMARY REVIEWS

instrument is usable in the coastal and nearshore environment giving rapid, reproducible data on sediment type and bottom dynamics that can be interpreted correctly by concurrent field observations (Day grab) and in the context of the knowledge of the studyarea. The data obtained in the preliminary attempt to extend the use of the Penetrometer to obtain field geotechnical data are presented in Table 3. The stations sampled are located along the SW (Station 3) to NE (Station 1) sediment bedload transport path (Shackley & Collins, 1984). When the liquid limit (LL) is plotted versus the plasticity index (PI) on the plasticity chart (Terzaghi et al., 1968), the three stations fall in sequence (1-3) along the A line

(established by Casagrande) and above that line (clays). On this chart Stations 1 and 2 occur in the zone of high plasticity, where slight erosion is expected (Gibbs, 1962) whilst Station 3 occurs in the zone of medium plasticity and highest resistance to erosion. When comparing these findings with the data on bottom dynamic type obtained with the Penetrometer (Table 3) Stations 1 and 2 are categorised as being in an area of transportation, and Station 3 as an area of accumulation . The similarity between these findings indicates that there is a good case for further detailed studies in order to extend the calibration table for the Penetrometer to include geotechnical data.

References COLLINS, M. B., C. B. PAlTIARATCHI, F. T. BANNER & G . K. FERENTINOS. 1980. The supply of sand to Swansea Bay. In (Collins, M. B. et al.; eds.), Industrialised embayments and their enuironmental problems ; A case study of Swansea Bay, Pergamon Press.

Oxford, 193-213. HAKANSON, L. & M. JANSSON. 1983. Principles of lake sedimentology, Springer-Verlag, 62-65 . SHACKLEY, S. E. 1982. The effects of dredged soil

disposal on the sublittoral non-consolidated sediments and benthic macroinvertebrate infauna of Swansea Bay, South Wales. Report To the Welsh Office Directorate of Environmental Engineering. November 1982, 131 pp. - - & M. B. COLLINS. 1984. Variations in sublittoral sediments and their associated macroinfauna in response to inner shelf processes; Swansea Bay, U.K. Sedimentology , 31, 793-804.

Geophysical assessment of sediment bioturbation in some Welsh estuaries S. E. Jones and C. F. Jago

School of Ocean Sciences, Marine Science Laboratories, Menai Bridge, Gwynedd, LL595EY

Estuaries are areas of intense human activity and considerable scientific interest. Their geological significance lies in their crucial participation in the sedimentary cycle (since they mark the transition from continental to marine environments) and, in particular, in their role as sediment traps (so that they are frequently sites of rapid sedimentation). They are therefore excellent environments in which to study modem processes in order to construct models of sedimentation that can aid interpretation of ancient sedimentary environments. As centres for commerce and recreation, estuaries are of obvious concern to the coastal engineer who frequently has to cope with the consequences of the same depositional processes that interest the sedimentologist. To the engineer , rapid sedimentation is usually a problem. Estuaries are also basins of mixing of fresh and salt water and as such play a vital part in the flux of materials, including pollutants, from land to sea . Since pollutants are often scavenged by sedimentary particles, the rate of exchange of material between the estuary and its bed (i.e . the rate of sedimentation) is again important. A

common therefore sediments across the

concern in many estuarine studies is the behaviour and fate of estuarine and , especially, the movement of sediments benthic boundary layer.

Organism/sediment interaction A key contribution to boundary layer processes is made by organisms. It is probable that such a contribution will be most felt in those environments where high densities of macrobenthic organisms occur. Estuaries are high stress environments from a biological point of view since the physico-chemical conditions are so frequently changing ; organisms have to tolerate large swings of salinity and temperature, and they must frequently survive drastic changes of sedimentary regime . Rather few species have adapted to such a fraught ecological niche. Nevertheless the few that have so adapted often prosper, so that an estuarine fauna consists of enormous numbers of individuals of relatively few species. This fauna must